This invention relates to electronic circuits for determining the position of a rotating shaft and more particularly to such circuits which are suitable for use in aircraft power generating systems wherein the rotating shaft is the shaft of a dynamoelectric machine which is capable of operating as a generator or a motor.
In airborne electrical power generation systems, it is desirable to have a single system which provides both the starter and generator functions. The weight savings on an airplane can be substantial when a dedicated starter is eliminated. For this reasin, electrical power systems which are capable of providing engine start functions can provide both cost and weight savings.
Variable speed constant frequency (VSCF) power generation systems are commonly used for aircraft applications. One type of VSCF system includes a variable speed generator which supplies DC power to a pair of DC link conductors. An inverter circuit receives DC power from the link conductors and produces a constant frequency AC output. The inherent simplicity and reliability of DC link VSCF systems has been established and it is desired to modify the existing designs to provide starter capability.
DC link converters commonly utilize a transistor bridge output circuit having a pair of series connected transistors in each output phase leg adapted to be connected between the DC link conductors, wherein a connection point between the transistors serves as an output power pole. One method used to convert such a DC link VSCF system to a starter/generator system includes the use of a contactor or set of contactors to turn the system around so that an external electrical power source supplies the inverter input power and the generator is connected to the inverter as a load. In order to drive the generator as a synchronous motor, the absolute position of the rotor shaft must be known so that the inverter output transistors can be switched at the appropriate times to provide the desired motor action.
Since aircraft generators operate at 20,000 rpm at 225.degree. C. with oil spray cooling, a rugged and reliable shaft position sensor is required. Common shaft position determining components such as resolvers, optical encoders, brush encoders and potentiometers will not survive in the harsh generator environment. A magnetic sensor/gear combination can provide a highly reliable position signal in such adverse environments. However, such systems provide relatively low resolution, do not provide relevant data at zero speed and do not known absolute position initially. It is therefore desirable to construct an absolute shaft position sensing circuit which includes a magnetic sensor/gear combination which overcomes these disadvantages.